1. Field of the Invention
The present invention relates to a brush apparatus used for, for example, an engine starter.
2. Description of the Related Art
In general, in order to achieve high-power and high-current in a direct current electric motor (DC electric motor) such as an engine starter, reduction of an internal electric resistance and upsizing of the electric motor are effective. As a measure for reducing the internal electric resistance, it is considered to increase a cross-sectional area of a conductor of a pigtail for the purpose of reduction of a current density and voltage drop of a lead wire which constitutes the pigtail. In order to do so, it is necessary to increase the thickness of the lead wire or to connect a plurality of the lead wires to a brush disclosed, for example, in JP-A-11-178285 and JP-A-2003-79093.
In order to increase the cross-sectional area of the conductor of the pigtail without changing the brush size for achieving the high-power and high-current of the DC electric motor in the related art, it is necessary to increase the thickness of the lead wire which constitutes the pigtail or to use a plurality of the lead wires. However, when the thickness of the lead wire increases, the finished outer diameter of the pigtail increases accordingly, and the dimensions of permissible wear from a surface of the brush, which comes into sliding contact with a commutator, to the lower end of a embedded portion of the pigtail decreases, which shortens the life of the brush. In addition, when the wear of the brush advances, the brush passes toward the commutator by a brush spring and hence moves downward in a brush holding member. However, the pigtail employing a thick lead wire has poor flexibility, which hinders the downward movement of the brush and causes a contact failure between the brush and the commutator, which may result in the voltage drop.
On the other hand, when two lead wires are embedded in the brush as shown in FIG. 4B in JP-A-2003-79093, since the positions where the two lead wires are embedded are different, the shape of a brush holder is constrained and hence a holding contact surface with respect to the brush decreases, which may result in lowering of the strength of the holding member and lowering of the holding stability. As shown in Patent Document JP-A-11-178285, when the lead wire is embedded on the opposite side of the surface which comes in sliding contact with the commutator, the brush is reliably held by the brush holder. Instead, however, there arises a necessity to avoid interference of the pigtail with respect to a bracket or the like on the outer periphery of the brush. Therefore, reduction of the brush height is necessary, and hence shortening of the brush life against the wear is unavoidable.